When a distillation operation is used in separating a mixture containing components that are very close in boiling points and volatilities, the number of plates required is very large and a large reflux ratio has to be used. Therefore, both the equipment cost and operating cost are very high. Furthermore, these costs increase greatly as the required product purity increases. Separations of ethylene-ethane mixtures, propylene-propane mixtures, styrene-ethyl benzene mixtures and p-xylene-m-xylene mixtures in producing high grade ethylene, propylene, styrene and p-xylene respectively are good examples. There is a great need for finding a better and more economical way of accomplishing the desired separations.
In a conventional fractional solidification process, a feed containing a crystallizing component and one or more impurities is brought into a two phase solid-liquid region to form a mixture containing crystals of the crystallizing component and a liquid mixture containing the impurities. An expensive scraped surface freezer is usually used in this operation. A centrifuge or a hydraulic washing column is then used to free the crystals from the mother liquor. Even though the crystals formed are usually very pure, it has been very difficult to produce a very high purity product of the crystallizing component because a complete separation of the crystals from the surrounding impure liquid phase is difficult. Furthermore, equipment cost of a conventional solidification process is usually very high.
It is important to distinguish the distillative freezing process of the present invention from a vacuum freezing desalination process, a vacuum crystallization process, a vacuum drying process, and a conventional desublimation process for removing a component from a gas mixture. In a vacuum freezing desalination process (also called an evaporative freezing process), only one component (water) vaporizes and the same component (water freezes. In a vacuum crystallization process or a vacuum drying process, only one component (solvent) evaporates while the other component (solute) crystallizes. In a conventional desublimation process, a gas mixture is cooleded down to a very low temperature to remove a crystallizing component (e.g. carbon dioxide or phthalic anhydride) by a desublimation operation and simply discharge the residual gas. In a distillative freezing process, two or more components are vaporized from a feed under a sufficiently reduced pressure and only one component freezes. The low pressure vapor formed in a distillative freezing process may be transformed completely into a condensed mass without pressurization by slightly lowering its temperature. The phase behaviour of a system to which a distillative freezing process applies is distinct from phase behaviors of systems to which these conventional processes apply.
The distillative freezing process described in U.S. Pat Nos. 4,218,893, 4,378,984, 4,451,237 and 4,433,558 are mostly concerned with basic and direct dry distillative freezing operations. The wet and dry distillative freezing process disclosed herein has several advantages over the direct dry approach: the drying-up temperature and pressure of the former are substantially higher than those of the latter and the former can even remove a major fraction of a low volatility impurity. Therefore, the field of application of the distillative freezing technology has been greatly broadened by the introduction of the wet and dry approach.